STOR-2462: Add ExternalSnapshotMetadata FeatureGate

[dobsonj]

https://issues.redhat.com/browse/STOR-2462

/cc @openshift/storage

[openshift-ci-robot]

@dobsonj: This pull request references STOR-2462 which is a valid jira issue.

Warning: The referenced jira issue has an invalid target version for the target branch this PR targets: expected the sub-task to target the "4.20.0" version, but no target version was set.

In response to this:

https://issues.redhat.com/browse/STOR-2462

/cc @openshift/storage

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[openshift-ci]

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In response to this:

https://issues.redhat.com/browse/STOR-2462

/cc @openshift/storage

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[openshift-ci]

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[openshift-ci]

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[dobsonj]

/retest

[openshift-ci]

@dobsonj: all tests passed!

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